This invention relates to an explosive composition, and a method of blasting with the explosive composition. In particular, this invention relates to an explosive composition comprised primarily of ammonium nitrate, fuel and a fluid, which is in the form of slurry, water gel, or emulsion explosive and which may be used in the surface mining of coal by cast blasting, the production of armourstone or riprap, free face rock blasting, and explosive stimulation of oil wells, gas wells, water wells and the like.
In the past it has been generally believed in the rock blasting art that for explosives comprised primarily of ammonium nitrate and fuel, higher velocities of propagation yield better blasting results, and it is well established that higher propagation velocities are the result of higher pressures in the chemical reaction zone of an exploding charge. Further, it has been generally believed that there is a minimum propagation velocity for commercial explosives of about 2000 m/s, below which the blasting action is unsatisfactory. Below this threshold, there are additional concerns about whether the reaction will go to completion, and whether, in light of the foregoing uncertainties, the charges in a series of holes would explode in about the same way. All of these concerns are based upon the desire to maximize the amount of useful work done by an explosive charge; incomplete explosions do not so maximize the useful work because of the unutilized energy left over in the unexploded portion or incompletely reacted ingredients. Indeed, such explosions often result in levels of ground vibration that are undesirably high, because the level of ground vibration produced by a charge of a given size increases greatly when its explosion has insufficient strength to break the rock to a free face. Consequently, typical commercial explosives are formulated and used so as to have propagation velocities of up to 3000-7000 m/sec, depending upon the rock involved.
There are many known blasting agent compositions and methods of using the same. Examples of prior patents for oil well stimulation include U.S. Pat. Nos. 3,630,284, 3,174,545 and 3,264,986. Examples of patents disclosing two or more component explosive compositions include U.S. Pat. Nos. 2,732,800, 3,342,132, 3,377,909, 3,462,324, Re Nos. 26,815, 3,474,729. Examples of annular lubricating through long conduits include U.S. Pat. Nos. 4,510,958 and 4,462,429. Various explosive compositions are disclosed in U.S. Pat. Nos. 4,287,010, 4,585,495, 4,619,721 and 4,714,503. An example of stemming a borehole is disclosed in U.S. Pat. No. 4,586,438.
Conventional commercial explosives, such as dynamite, pentolite and ANFO, as normally used, explode by detonation, and are therefore known as high explosives. Essentially, detonation occurs where the reaction zone and its high pressure wave propagate at a velocity greater than the velocity of sound. "High order" detonation occurs where the chemical reaction in the reaction zone goes essentially to completion before lateral expansion occurs. "Low order" detonation occurs where there is lateral expansion of the material in the chemical reaction zone prior to the chemical reaction being substantially completed.
The disadvantage of "high order" detonation, however, is that the level of pressure associated with the pressure wave is typically above the crushing strength of the material being blasted. Consequently, "high order" blasting tends to utilize significant amounts of energy in crushing the rock and producing fines. The energy used to crush the rock is essentially wasted. Furthermore, when such charges are used to stimulate wells, the zone of crushed rock can block the desired extension of gas-pressured fractures out into the formation, can make post-shot cleanout more difficult, and finally can block production of the completed well.
The disadvantage of "low order" detonation is that with a detonation velocity below about 1000 m/sec in commercial blasting agents having a density of 0.85 or greater, it has been noted that the result has been unstable rates of detonation, with incomplete chemical reaction and poor blasting results. Explosives Engineering Vol. 4, No. 1 P.5, May/June 1986 describes the unsatisfactory blasting behaviour of an ANFO explosive that had become wet during loading, and which had an explosive velocity of 623 m/sec. The author suggests that when such behaviour occurs, the explosive efficiency of ANFO suffers greatly. The author teaches how to maintain high velocities by placing cartridges of a more sensitive explosive every few feet within the charge.
Black blasting powder, which has a typical explosive propagation velocity of about 400 m/sec, explodes by a different explosive mechanism, namely, by explosive deflagration. Explosive deflagration is not propagated by a shock wave, but is rather propagated by convective flow of hot gases from ignited grains to the interstices between unignited grains, which causes further ignition of said grains. However, black blasting powder is too low in energy density, too dangerous, too expensive and too difficult to utilize to be a viable modern commercial blasting explosive. Explosive deflagration by convective flow through interstices cannot work in conventional high density blasting agents because they are not sufficiently flammable and because their interstices are either too small or not present at all.